The advantages of working a metal in its solid state, the equilibrium state under most working conditions, rather than its liquid state are well known. The enhanced reactivity of the metal in the liquid state makes it prone to reaction with the atmosphere or mold, die or furnace elements, resulting in the formation of solid inclusions and/or incorporation of dissolved gases into the melt. Processes involving molten metal also necessarily involve phase transformation associated with solidification shrinkage, evolution of dissolved gases and a number of casting defects.
On the other hand, working metal in the solid state requires a large amount of energy to deform the metal, necessitating heavy and expensive machinery. It is known to extrude a material, typically a soft metal (e.g., aluminum, copper, magnesium, zinc, silver and alloys thereof) in the form of a continuous cable, tube or ribbon through a die by maintaining frictional engagement of the material with a passageway defined by driving and non-driving surfaces, such that frictional drag maintains extrusion pressure and urges the material through the die ("frictional extrusion"). This process has been typically used for preparing continuous lengths of cable or tubing. The reader is directed to the prior art on continuous extrusion for more specific details, e.g., GB 1,370,894, GB 1,566,152, and GB 1,590,766.
It is desirable to develop an extrusion process capable of use for preparing massive structures of non-uniform cross-section because the process is relatively inexpensive in comparison to conventional metal-working processes, such as forging, and provides inherently higher quality materials than some less expensive casting processes. However, extrusion of large articles with non-uniform cross-sectional areas results in variation of extrusion processing conditions, such as velocity and pressure, along the extrusion pathway. Such processing variations can result in increased porosity and/or inclusions, as well as other structural defects in the final product.
In conventional extrusion processes, the surface over which extrusion occurs is small and the extrusion pressure is correspondingly small, as well. When it is desired to extrude a metal into a die chamber of increased complexity, the material must move (be extruded) over a large regions of varying cross-sectional area. The forces on the material are very large. Hence, conventional continuous extrusion processes are not readily adaptable to the preparation of large metal pieces.
Frictional extrusion processes have addressed the problem of extruding product (typically large bore tubing) having a final dimension greater than the largest dimension of the feed material (a controlling parameter in extrusion processes). GB 1,507,303 discloses an apparatus for extruding a product having a dimension greater than the largest dimension of the feed material by gradually increasing the passage dimension in the direction from the inlet end to the outlet end. GB 1,566,152 discloses the use of multiple feeds into an intermediate chamber from which one or more die orifices may extend. U.S. Pat. No. 5,152,163 discloses the production of thin-walled large cross-section products extruded with the use of mixer plates and feeder blocks. None of the prior art references have addressed the unique processing problems related to forming discreet complex articles.
GB 1,504,890 discloses continuous extrusion of shaped articles, whose cross sectional areas are substantially uniform. Further, because the mold is in a carousel housed within the driving or non-driving surfaces of the apparatus, the size of the shaped articles is necessarily small and the shape is rather simple.